This invention relates to fluid ejection devices.
Ink jet printers employ pens having print heads that reciprocate relative to a media sheet and expel droplets through an array of nozzles onto the sheet to generate a printed image or pattern. The print heads have arrays of small orifices through which ink is expelled to generate a swath of a printed image.
Two important measures of printer performance are speed and print quality, which typically trade off with each other so that maximizing one compromises the other. The print speed is primarily limited by the scan velocity and by the length of the nozzle array (i.e. the width of a single printed swath). The print quality is primarily limited by the resolution or spacing of nozzles on the print head. For a given array length, the print quality may be maximized by printing multiple overlapping swaths to multiply the array""s resolution, with the droplets of each swath filling the spaces between the droplets of other swaths. To maximize print speed, single passes are used.
Developments have led to higher resolution print heads that improve print quality without a speed compromise. However, these developments are limited by physical constraints on the miniaturization of print head components. To provide additional improvements in performance, larger print heads having longer arrays may be used. However, as print heads are made larger, they become more expensive. Beyond the proportional cost per unit area of semiconductor material, larger print head chips result in greater wafer edge losses, and other costs associated with larger chips. For instance, a single defect on a wafer ruins a larger percentage of that wafer""s chips.
To avoid the costs associated with larger chips, a multiple chip print head may be employed, either with two or more print head chips of moderate size arranged on a common substrate, or with separate print heads installed in a printer. Such print heads are installed with their nozzle arrays parallel, and offset from each other along the media feed axis to cover adjacent swaths to generate a larger swath.
Such arrangements suffer from an alignment problem that can create a visible artifact where the adjacent swaths join each other at a seam. With high resolution print heads, a small misalignment at the limits of manufacturing capability can be several times the nozzle pitch, or at least a major fraction of the nozzle pitch. When separately replaceable print heads are used, the misalignment can be even greater. Electronic measures may permit correction of multiple-nozzle errors by slightly overlapping the swaths, and disabling the extra overlapping nozzles. However, this technique must tolerate an alignment error of up to one-half the nozzle pitch, leading to a possible gap or overlap of that amount. Such errors are visible as a light or dark band on the printed page.
Accordingly, the present invention provides a fluid ejection device with a body defining an array of nozzles. The nozzles are preferably arranged in an array along an array axis. The array has a first portion in which the nozzles are spaced apart along the array axis by a first pitch, and a second portion in which the nozzles are spaced apart by a different second pitch. In one embodiment, the first and second portions may be configured of approximately the same size. In another embodiment, the array may have a third portion between the first and second portions with a third pitch different from the first and second pitch.
An assembly may include two or more of such fluid ejecting devices or print heads. The second portion of one print head may be aligned with the first portion of the other print head. Such an assembly may be operated by determining an aligned pair of nozzles, and disabling the nozzles extending beyond each member of the pair and disabling one of the pair.
The present invention also includes printers that incorporate these types of fluid ejection devices and related methods of operating such a printer.